Source lines for NAND memory devices

ABSTRACT

A source line is formed by forming a source slot in a bulk insulation layer overlying a substrate to expose a portion of a substrate within the source slot, where the exposed portion of the substrate includes source regions of select gates associated with two or more columns of serially-connected floating-gate transistors formed on the substrate. A layer of epitaxial silicon is grown on the exposed portion so as to partially fill the source slot. A conductive layer is formed on the bulk insulation layer and on the layer of epitaxial silicon so as to substantially fill an unfilled portion of the source slot. The conductive layer is removed from a surface of the bulk insulation layer.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of application Ser. No. 10/855,844,titled “SOURCE LINES FOR NAND MEMORY DEVICES,” filed May 27, 2004(pending), which application is assigned to the assignee of the presentinvention and the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and inparticular the present invention relates to source lines for NAND memorydevices.

BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal storage areas incomputers. The term memory identifies data storage that comes in theform of integrated circuit chips. In general, memory devices contain anarray of memory cells for storing data, and row and column decodercircuits coupled to the array of memory cells for accessing the array ofmemory cells in response to an external address.

One type of memory is a non-volatile memory known as flash memory. Aflash memory is a type of EEPROM (electrically-erasable programmableread-only memory) that can be erased and reprogrammed in blocks. Manymodern personal computers (PCs) have their BIOS stored on a flash memorychip so that it can easily be updated if necessary. Such a BIOS issometimes called a flash BIOS. Flash memory is also popular in wirelesselectronic devices because it enables the manufacturer to support newcommunication protocols as they become standardized and to provide theability to remotely upgrade the device for enhanced features.

A typical flash memory comprises a memory array that includes a largenumber of memory cells arranged in row and column fashion. Each of thememory cells includes a floating-gate field-effect transistor capable ofholding a charge. The cells are usually grouped into blocks. Each of thecells within a block can be electrically programmed on an individualbasis by charging the floating gate. The charge can be removed from thefloating gate by a block erase operation. The data in a cell isdetermined by the presence or absence of the charge on the floatinggate.

NOR and NAND flash memory devices are two common types of flash memorydevices, so called for the logical form the basic memory cellconfiguration in which each is arranged. Typically, for NOR flash memorydevices, the control gate of each memory cell of a row of the array isconnected to a word-select line, and the drain region of each memorycell of a column of the array is connected to a bit line. The memoryarray for NOR flash memory devices is accessed by a row decoderactivating a row of floating gate memory cells by selecting theword-select line coupled to their gates. The row of selected memorycells then place their data values on the column bit lines by flowing adiffering current, depending upon their programmed states, from acoupled source line to the coupled column bit lines.

The array of memory cells for NAND flash memory devices is also arrangedsuch that the control gate of each memory cell of a row of the array isconnected to a word-select line. However, each memory cell is notdirectly coupled to a column bit line by its drain region. Instead, thememory cells of the array are arranged together in strings (often termedNAND strings), typically of 32 each, with the memory cells coupledtogether in series, source to drain, between a source line and a columnbit line. The memory array for NAND flash memory devices is thenaccessed by a row decoder activating a row of memory cells by selectingthe word-select line coupled to a control gate of a memory cell. Inaddition, the word-select lines coupled to the control gates ofunselected memory cells of each string are driven to operate theunselected memory cells of each string as pass transistors, so that theypass current in a manner that is unrestricted by their stored datavalues. Current then flows from the source line to the column bit linethrough each series coupled string, restricted only by the selectedmemory cells of each string. This places the current-encoded data valuesof the row of selected memory cells on the column bit lines.

The source line should have a low resistance to keep the voltage dropalong the source line low so as to avoid memory device failure.Moreover, the source line should have low current leakage when biasingthe source line. Source lines are often made from polysilicon, whichnormally satisfies current-leakage requirements, but has a relativelyhigh resistance that can cause excessive voltage drops that may lead todevice failure. Connecting each NAND string to a metal line formed on alayer different from that containing the NAND strings using apolysilicon contact can reduce resistance. However, this method requiresa patterned masking step for forming the metal line and anotherpatterned masking step for forming the contacts. Forming a metal line ona different level than the polysilicon source line and periodicallyconnecting the source line and metal line by forming contacts betweenthe source and metal lines can reduce the resistance compared to an allpolysilicon source line. However, each contact consumes silicon realestate.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternative source lines for NAND flash memory devices.

SUMMARY

The above-mentioned problems with source lines for NAND flash memorydevices and other problems are addressed by the present invention andwill be understood by reading and studying the following specification.

For one embodiment, the invention provides a method of forming a portionof a NAND memory including forming a source line. Forming the sourceline includes forming a source slot in a bulk insulation layer overlyinga substrate to expose a portion of the substrate within the source slot,where the exposed portion of the substrate includes source regions ofselect gates associated with two or more columns of serially-connectedfloating-gate transistors formed on the substrate. A layer of epitaxialsilicon is grown on the exposed portion so as to partially fill thesource slot. A conductive layer is formed on the bulk insulation layerand on the layer of epitaxial silicon so as to substantially fill anunfilled portion of the source slot. The conductive layer is removedfrom a surface of the bulk insulation layer.

The invention further provides methods of varying scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a memory system according to anembodiment of the present invention.

FIG. 2 is a schematic of a NAND memory array according to anotherembodiment of the present invention.

FIG. 3 is a cross-sectional view of a portion of a memory array during astage of fabrication after several processing steps have occurredaccording to another embodiment of the present invention.

FIGS. 4A-4D are cross-sectional views of a portion of a memory arrayduring various stages of fabrication according to another embodiment ofthe invention.

FIGS. 5A-5C are cross-sectional views of a portion of a memory arrayduring various stages of fabrication according to yet another embodimentof the invention.

FIGS. 6A-6C are cross-sectional views of a portion of a memory arrayduring various stages of fabrication according to still anotherembodiment of the invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The terms wafer orsubstrate used in the following description include any basesemiconductor structure. Both are to be understood as includingsilicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI)technology, thin film transistor (TFT) technology, doped and undopedsemiconductors, epitaxial layers of a silicon supported by a basesemiconductor structure, as well as other semiconductor structures wellknown to one skilled in the art. Furthermore, when reference is made toa wafer or substrate in the following description, previous processsteps may have been utilized to form regions/junctions in the basesemiconductor structure, and terms wafer or substrate include theunderlying layers containing such regions/junctions. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof.

FIG. 1 is a simplified block diagram of a memory system 100 according toan embodiment of the invention. Memory system 100 includes an integratedcircuit NAND flash memory device 102 that includes an array of flashmemory cells 104, an address decoder 106, row access circuitry 108,column access circuitry 110, control circuitry 112, Input/Output (I/O)circuitry 114, and an address buffer 116. Memory system 100 includes anexternal microprocessor 120, or memory controller, electricallyconnected to memory device 102 for memory accessing as part of anelectronic system. The memory device 102 receives control signals fromthe processor 120 over a control link 122. The memory cells are used tostore data that are accessed via a data (DQ) link 124. Address signalsare received via an address link 126 that are decoded at address decoder106 to access the memory array 104. Address buffer circuit 116 latchesthe address signals. The memory cells are accessed in response to thecontrol signals and the address signals. It will be appreciated by thoseskilled in the art that additional circuitry and control signals can beprovided, and that the memory device of FIG. 1 has been simplified tohelp focus on the invention.

FIG. 2 illustrates a NAND memory array 200 as a portion of memory array104 in accordance with another embodiment of the invention. As shown inFIG. 2, the memory array 200 includes word lines 202 ₁ to 202 _(N) andintersecting local bit lines 204 ₁ to 204 _(M). For ease of addressingin the digital environment, the number of word lines 202 and the numberof bit lines 204 are each some power of two, e.g., 256 word lines 202 by4,096 bit lines 204. The local bit lines 204 are coupled to global bitlines (not shown in FIG. 2) in a many-to-one relationship.

Memory array 200 includes NAND strings 206 ₁ to 206 _(M). Each NANDstring includes floating gate transistors 208 ₁ to 208 _(N), eachlocated at an intersection of a word line 202 and a local bit line 204.The floating gate transistors 208 represent non-volatile memory cellsfor storage of data. The floating gate transistors 208 of each NANDstring 206 are connected in series source to drain between a sourceselect gate 210, e.g., a field effect transistor (FET), and a drainselect gate 212, e.g., an FET. Each source select gate 210 is located atan intersection of a local bit line 204 and a source select line 214,while each drain select gate 212 is located at an intersection of alocal bit line 204 and a drain select line 215.

A source of each source select gate 210 is connected to a common sourceline 216. The drain of each source select gate 210 is connected to thesource of the first floating gate transistor 208 of the correspondingNAND string 206. For example, the drain of source select gate 210 ₁ isconnected to the source of floating gate transistor 208 ₁ of thecorresponding NAND string 206 ₁. A control gate 220 of each sourceselect gate 210 is connected to source select line 214. It is common fora common source line to be connected between source select gates forNAND strings of two different NAND arrays. As such, the two NAND arraysshare the common source line.

The drain of each drain select gate 212 is connected to the local bitline 204 for the corresponding NAND string at a drain contact 228. Forexample, the drain of drain select gate 212 ₁ is connected to the localbit line 204 ₁ for the corresponding NAND string 206 ₁ at drain contact228 ₁. The source of each drain select gate 212 is connected to thedrain of the last floating gate transistor 208 _(N) of the correspondingNAND string 206. For example, the source of drain select gate 212 ₁ isconnected to the drain of floating gate transistor 208 _(N) of thecorresponding NAND string 206 ₁. It is common for two NAND strings toshare the same drain contact.

Typical construction of floating gate transistors 208 includes a source230 and a drain 232, a floating gate 234, and a control gate 236, asshown in FIG. 2. Floating gate transistors 208 have their control gates236 coupled to a word line 202. A column of the floating gatetransistors 208 is a NAND string 206 coupled to a given local bit line204. A row of the floating gate transistors 208 are those transistorscommonly coupled to a given word line 202.

FIG. 3 depicts a portion of a memory array during a stage of fabricationafter several processing steps have occurred according to an embodimentof the present invention. Formation of the structure depicted in FIG. 3is well known and will not be detailed herein. In general, however,source select gates 310 are formed on a substrate 312, e.g., of silicon,such as monocrystalline silicon. For one embodiment, source select gates310 share a source/drain region 313 formed on substrate 312. Each ofsource select gates 310 further includes a source/drain region 314formed on substrate 312, a gate dielectric 316 formed on substrate 312between source drain regions 313 and 314, and a control gate 318 formedon gate dielectric 316, as shown in FIG. 3.

Although not shown in FIG. 3, source select gates 310 are each connectedto a NAND string, as described above and shown in FIG. 2. The respectiveNAND strings are connected to drain select gates formed on substrate 312(not shown in FIG. 3), as described above and shown in FIG. 2. A bulkinsulation layer (or dielectric layer) 315 is formed overlying substrate312, source select gates 310, the NAND strings, the drain select gates,and exposed surfaces of the substrate adjacent the source select gates.One example for the insulation layer 315 would be a doped silicateglass, such as BSG (borosilicate glass), PSG (phosphosilicate glass),and BPSG (borophosphosilicate glass).

Insulation layer 315 is patterned, e.g., with a deep ultraviolet (DUV)photolithography process, to define a source slot 320. Source slot 320is etched into insulation layer 315 down to substrate 312 between selectgates 310 to expose a portion of substrate 312, i.e., the sharedsource/drain region 313, between select gates 310. Source slot 320 istrench shaped and extends perpendicularly to the plane of FIG. 3 so asto span two or more columns of memory cells (or NAND strings). That is,source slot 320 extends between two or more select gates respectivelyconnected to the two or more NAND strings. For one embodiment, sourceslot 320 spans an entire memory array, e.g., source slot 320 extendsbetween select gate 210 ₁ and 210 _(M) of memory array 200 of FIG. 2.

FIGS. 4A-4D generally depict a method of forming a source line 416 inthe source slot 320 (shown in FIG. 4D) of the structure of FIG. 3. Forone embodiment, source line 416 is a common source line as described forcommon source line 216 of FIG. 2. That is, two or more source selectgates respectively coupled to two or more NAND strings are coupled tosource line 416.

A first conductive layer (or polysilicon layer) 420 is formed over thestructure of FIG. 3 so that polysilicon completely fills source slot 320and contacts the exposed portion of substrate 312, as shown in FIG. 4A,using a suitable deposition technique, such as chemical vapor deposition(CVD). For one embodiment, deposition of polysilicon layer 420 includesin situ conductive doping of the polysilicon, i.e., dopant is added tothe polysilicon while polysilicon layer 420 is being formed. Polysiliconlayer 420 is etched back and recessed into source slot 320, leaving aportion of source slot 320 above the recessed polysilicon layer 420unfilled, as shown in FIG. 4B. For one embodiment, the recessedpolysilicon layer 420 of FIG. 4B is about 1000 to about 3000 angstromsthick. A dry etch, such as a plasma etch, or a wet etch, e.g., usingTetramethylammonium hydroxide (TMAH), can be used to etch backpolysilicon layer 420.

A second conductive layer 440 having a higher electrical conductivitythan polysilicon layer 420 is formed on the structure of FIG. 4B, asshown in FIG. 4C, and can be formed using standard metallizationprocedures. For example, for one embodiment, second conductive layer 440is formed by depositing a barrier layer 442, e.g., a refractory metalnitride, such as titanium nitride (TiN) or tungsten nitride (WN_(x)), oninsulation layer 315 and recessed polysilicon layer 420, e.g., usingCVD. An adhesion layer 444, e.g., a metal layer, such as titanium (Ti)is deposited on barrier layer 442, e.g., using CVD. A metal layer 446,such as tungsten (W), is deposited on the adhesion layer 444, e.g.,using CVD. In addition to CVD, physical vapor deposition (PVD), e.g.,sputtering, can be used. For another embodiment, second conductive layer440 is refractory metal silicide layer overlying insulation layer 315and polysilicon plug 430. The metals of chromium (Cr), cobalt (Co),hafnium (Hf), molybdenum (Mo), niobium (Nb), tantalum (Ta), titanium(Ti), tungsten (W), vanadium(V) and zirconium (Zr) are generallyrecognized as refractory metals.

For one embodiment, chemical mechanical planerization (CMP) is performedto produce the structure of FIG. 4D. Specifically, second conductivelayer 440 is removed from a surface of insulation layer 315 so thatconductive layer 440 is substantially flush with insulation layer 315and substantially fills the unfilled portion of source slot 320.Recessed polysilicon layer 420 and conductive layer 440 form the sourceline 416.

FIGS. 5A-5C generally depict a method of forming a source line 516 inthe source slot 320 (shown in FIG. 5C) of the structure of FIG. 3according to another embodiment of the present invention. A firstconductive layer (or polysilicon layer) 520 is formed over the structureof FIG. 3, as shown in FIG. 5A using a suitable deposition technique,such as chemical vapor deposition (CVD). For one embodiment, depositionof polysilicon layer 520 includes in situ conductive doping of thepolysilicon, i.e., dopant is added to the polysilicon while polysiliconlayer 520 is being formed. As shown in FIG. 5A, polysilicon layer 520follows the contour of slot 320, rather than completely filling slot320, as shown in FIG. 4A. Specifically, polysilicon layer 520 coats theinterior of slot 320, i.e., polysilicon layer 520 coats the sidewalls ofslot 320 and the exposed portion of substrate 312 that forms the bottomof slot 320. This eliminates the etch-back and recessing step of FIG.4B. For one embodiment, polysilicon layer 520 is about 200 angstromsthick.

A second conductive layer 540 having a higher electrical conductivitythan polysilicon layer 520 is formed on the structure of FIG. 5A, asshown in FIG. 5B, and can be formed using standard metallizationprocedures. For example, for one embodiment, second conductive layer 540is formed by depositing a barrier layer 542, e.g., a refractory metalnitride, such as titanium nitride (TiN) or tungsten nitride (WN_(x)), oninsulation layer 315 and polysilicon layer 520, e.g., using CVD. Anadhesion layer 544, e.g., a metal layer, such as titanium (Ti), isdeposited on barrier layer 542, e.g., using CVD. A metal layer 546, suchas tungsten (W), is on the adhesion layer 544, e.g., using CVD. Inaddition to CVD, physical vapor deposition (PVD), e.g., sputtering, canbe used. For another embodiment, second conductive layer 540 isrefractory metal silicide layer overlying insulation layer 315 andpolysilicon layer 520.

For one embodiment, CMP is performed to produce the structure of FIG.5C. Specifically, the second conductive layer 540 and polysilicon layer520 are removed from a surface of insulation layer 315 so that secondconductive layer 540 is substantially flush with insulation layer 315and substantially fills an unfilled portion of source slot 320.Polysilicon layer 520 contains second conductive layer 540 and separatessecond conductive layer 540 from insulation layer 315 and the exposedportion of substrate 312. Polysilicon layer 520 and conductive layer 540form the source line 516.

FIGS. 6A-6C generally depict a method of forming a source line 616 inthe source slot 320 (shown in FIG. 6C) of the structure of FIG. 3according to another embodiment of the present invention. Referring toFIG. 6A, an epitaxial silicon layer 620 is selectively grown, ordeposited, on the exposed monocrystalline silicon of substrate 312within slot 320 so as to leave a portion of source slot 320 aboveepitaxial silicon layer 620 unfilled. For one embodiment, epitaxialsilicon layer 620 is conductively doped, e.g., using ion implantationafter its formation. Epitaxial deposition and ion implantation are wellunderstood in the art and will not be discussed further here.Selectively growing epitaxial silicon layer 620 on the exposed substrateeliminates the etch-back and recessing step of FIG. 4B because epitaxialsilicon layer 620 grows generally upward from the bottom of source slot320. For one embodiment, epitaxial silicon layer 620 is about 500 toabout 1000 angstroms thick.

A conductive layer 640 having a higher electrical conductivity thanepitaxial silicon layer 620 is formed on the structure of FIG. 6A, asshown in FIG. 6B, and can be formed using standard metallizationprocedures. For example, for one embodiment, conductive layer 640 isformed by depositing a barrier layer 642, e.g., a refractory metalnitride, such as titanium nitride (TiN) or tungsten nitride (WN_(x)), oninsulation layer 315 and epitaxial silicon layer 620, e.g., using CVD.An adhesion layer 644, e.g., a metal layer, such as titanium (Ti), isdeposited on barrier layer 642, e.g., using CVD. A metal layer 646, suchas tungsten (W), is deposited on the adhesion layer 644, e.g., usingCVD. In addition to CVD, physical vapor deposition (PVD), e.g.,sputtering, can be used. For another embodiment, conductive layer 640 isrefractory metal silicide layer overlying insulation layer 315 andepitaxial silicon layer 620.

For one embodiment, CMP is performed to produce the structure of FIG.6C. Specifically, the conductive layer 640 is removed from a surface ofinsulation layer 315 so that conductive layer 640 substantially fillsthe unfilled portion of source slot 320 and is substantially flush withinsulation layer 315. Epitaxial silicon layer 620 and conductive layer640 form the source line 616.

CONCLUSION

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A method of forming a portion of a NAND memory array, the methodcomprising: forming a source line, wherein forming the source linecomprises: forming a source slot in a bulk insulation layer overlying asubstrate to expose a portion of the substrate within the source slot,wherein the exposed portion of the substrate includes source regions ofselect gates associated with two or more columns of serially-connectedfloating-gate transistors formed on the substrate; growing a layer ofepitaxial silicon on the exposed portion so as to partially fill thesource slot; forming a conductive layer on the bulk insulation layer andon the layer of epitaxial silicon so as to substantially fill anunfilled portion of the source slot; and removing the conductive layerfrom a surface of the bulk insulation layer.
 2. The method of claim 1,further comprising doping the layer of epitaxial silicon after itsformation.
 3. The method of claim 2, wherein doping the layer ofepitaxial silicon comprises ion implantation.
 4. The method of claim 1,wherein forming the conductive layer comprises: forming a barrier layeron the layer of epitaxial silicon and the bulk insulation layer; forminga first metal layer on the barrier layer; and forming a second metallayer on the first metal layer.
 5. The method of claim 4, wherein thebarrier layer, first metal layer, and second metal layer arerespectively of titanium nitride, titanium, and tungsten.
 6. The methodof claim 1, wherein the layer of epitaxial silicon is about 500angstroms to about 1000 angstroms.
 7. A method of fabricating a portionof a memory device, comprising: forming two or more columns of memorycells of a memory array, wherein forming each column of memory cellscomprises forming a plurality of serially-connected floating-gatetransistors overlying a substrate; forming a first source select gateoverlying the substrate and coupled to a drain of a source of a firstfloating-gate transistor of a first column of the two or more columns ofmemory cells; forming a second source select gate overlying thesubstrate and coupled to a drain of a source of a first floating-gatetransistor of a second column of the two or more columns of memorycells; forming a layer of dielectric material overlying the first andsecond source select gates, the two or more columns of memory cells, andexposed surfaces of the substrate adjacent the first and source selectgates; patterning the layer of dielectric material to define a sourceslot; etching the source slot into the layer of dielectric material toexpose a portion of the substrate within the source slot, wherein theexposed portion of the substrate includes source regions of the firstand second source select gates; growing a layer of epitaxial silicon onthe exposed portion; forming a conductive layer on the layer ofdielectric material and on the layer of epitaxial silicon; and removingthe conductive layer from a surface of the layer of dielectric material.8. The method of claim 7, further comprising doping the layer ofepitaxial silicon after its formation.
 9. The method of claim 7, whereinforming the conductive layer comprises: forming a barrier layer on thelayer of epitaxial silicon and the layer of dielectric material; forminga first metal layer on the barrier layer; and forming a second metallayer on the first metal layer.
 10. The method of claim 9, wherein thebarrier layer, first metal layer, and second metal layer arerespectively of titanium nitride, titanium, and tungsten.
 11. The methodof claim 7, wherein the layer of epitaxial silicon is about 500angstroms to about 1000 angstroms.
 12. The method of claim 7, whereinthe conductive layer has a higher electrical conductivity than the layerof epitaxial silicon.
 13. A method of forming a portion of a NAND memoryarray, the method comprising: forming a source line, wherein forming thesource line comprises: forming a source slot in a bulk insulation layeroverlying a substrate to expose a portion of the substrate within thesource slot, wherein the exposed portion of the substrate includessource regions of select gates associated with two or more columns ofserially-connected floating-gate transistors formed on the substrate;growing a layer of epitaxial silicon on the exposed portion so as topartially fill the source slot; forming a conductive layer on the bulkinsulation layer and on the layer of epitaxial silicon so as tosubstantially fill an unfilled portion of the source slot; and removingthe conductive layer from a surface of the bulk insulation layer;wherein the conductive layer has a higher electrical conductivity thanthe layer of epitaxial silicon.
 14. The method of claim 13, furthercomprising doping the layer of epitaxial silicon after its formation.15. The method of claim 14, wherein doping the layer of epitaxialsilicon comprises ion implantation.
 16. The method of claim 13, whereinthe conductive layer is a refractory metal silicide layer.
 17. Themethod of claim 13, wherein forming the conductive layer comprises:forming a barrier layer on the layer of epitaxial silicon and the bulkinsulation layer; forming a first metal layer on the barrier layer; andforming a second metal layer on the first metal layer.
 18. The method ofclaim 17, wherein the barrier layer, first metal layer, and second metallayer are respectively of titanium nitride, titanium, and tungsten. 19.The method of claim 13, wherein the layer of epitaxial silicon is about500 angstroms to about 1000 angstroms.
 20. The method of claim 17, wherein the conductive layer is removed from the surface of the bulkinsulation layer using CMP.